Document how to use Qt Metatype system with types in C++ namespaces

Using custom namespaced types in signals, slots, invokables requires special care. From QTBUG-73761 :

This is a known limitation of the current QMetaObject/QMetaType system, which do lookup by name.

So the name (seen by moc) need to be be written the same in the Q_DECLARE_METATYPE, and in the Q_PROPERTY, and in the class definition.

In Qt, we usually therefore fully qualify all types in Q_INVOKABLE, slot, and Q_PROPERTY signatures, as well as in qmlRegisterTypeXXX calls. ANyhow, this is only documented for Q_DECLARE_METATYPE:

If MyStruct is in a namespace, the Q_DECLARE_METATYPE() macro has to be outside the namespace:

Such advice should be added to the documentation of Q_INVOKABLE, Q_PROPERTY ... too. An overview section about this might be added e.g. to https://doc.qt.io/qt-5/metaobjects.html .